There are different solutions for the implementation of electric and magnetic field sensors at frequencies below just a few kilohertz. These sensors cover ELF, SLF and ULF bands which range from 3 Hz to 3 kHz and which we will refer to hereinafter as low frequencies (LF). The electric field (E) sensors are based on capacitive elements such as for example parallel and flat plate capacitors. The magnetic field (B) sensors are based on windings wherein the field B induces a current by virtue of Faraday's Law. In order to make this winding immune to E field, the winding is shielded by means of a metal screen. This screen tends to have an opening or gap whose function is to prevent the circulation of currents induced by the B field in the screen.
There are some probe designs which provide for measurement of LF E and B fields simultaneously. These sensors consist of a coil with two diametrically opposite gaps wherein balancing elements (baluns) are disposed to transfer the voltages measures in the two gaps to a circuit for adding and subtracting currents whose output signals are proportional to the E and B fields.
The drawback of this design, in addition to the added complexity entailed by the baluns and the signal addition and subtraction circuit, is that its response to the B field is only noticeable at frequencies far above the LF range. This is due to the fact that the sensor consists of a single coil.
Finally, there are solutions for measuring the LF E and B fields which propose increasing the sensitivity to the B field by means of a winding with several turns shielded using a screen with two gaps.
Specifically, these are isotropic sensors of magnetic B and electric E fields, which comprise three sensor units, each sensor unit comprising a winding of lead wire surrounded by a conducting screen, the screen being interrupted in two first diametrically opposite zones in such a way that two independent screen segments are defined, there being in one of said zones the terminals for measuring the currents induced by the magnetic field in the lead wire winding and in the other zone the terminals for measuring the voltage induced in the screen by the electric field.
This shielding acts as an E field sensor in a similar way to that of the coil with two gaps described in the preceding paragraph. A sensor like this one, considered to be the closest state of the art, is shown in FIG. 1.
However, these designs do not permit simultaneous measurement of E and B as they make use of moving or mechanical parts (switches) which must be actuated by the sensor's operator. They also present the drawback of the electric sensor's immunity to B fields being very limited in field amplitude and frequency. This is due to the fact that in these sensors the B field induces a current which travels across the perimeter of the shield. This in turn produces a displacement current in the gap where it is intended to measure E, which gives rise to a voltage drop associated to the B field and not E.